The present invention relates to an electronic balance having a built-in calibration weight.
In general, a conventional electronic balance includes a load detecting unit having a combination of an electromagnetic force balance mechanism and a load sensor in a housing. The load detecting unit detects a load applied to a sensitive member in proportion to a load of a sample placed on a sample plate.
In the electronic balance having the load detecting unit as described above, there may be deviation (error) in a measured result (displayed value) according to a change in an environmental condition, such as a temperature and an air pressure, and lapse of time. Therefore, in order to accurately measure a weight of a sample all the time, it is required to carry out an automatic measurement sensitivity calibration (hereinafter, simply referred to as “sensitivity calibration”). Normally, a weight for the sensitivity calibration is disposed in the electronic balance, and the electronic balance includes a sensitivity calibration mechanism for calibrating the sensitivity by applying the weight.
FIG. 6 is a view showing a sensitivity calibration mechanism in a conventional electronic balance disclosed in Japanese Patent Publication (Kokai) No. 2000-74730. As shown in FIG. 6, in the sensitivity calibration mechanism, a sensitive member 3 is an upright member extending vertically, and an upper end portion thereof protrudes outside through a hole provided in a housing 1. A sample plate 2 is supported on a leading end of the sensitive member 3. A cap-shape weight receptacle 8 is disposed at the vicinity of an upper end portion of the sensitive member 3, for example, right under a supporting portion of the sample plate 2.
The sample plate 2 is an integrated part having a disc shape as a whole, and has a peripheral portion bent downwardly and a hole for inserting the leading end of the sensitive member 3 at a central portion of a lower surface thereof. A calibration weight 4 having a disc shape with a center thereof arranged coaxially with an axis of the sensitive member 3 is disposed in a space between the lower surface of the sample plate 2 and an upper surface of the housing 1.
When a sliding cam mechanism 6 drives a vertically moving bar 5 to push the calibration weight 4 upwardly, the vertically moving bar 5 and a weight holding member 7 hold the calibration weight 4, and the load of the calibration weight 4 is removed from the load detecting unit. When the vertically moving bar 5 is lowered, the load of the calibration weight 4 is applied to the sensitive member 3, so that the calibration is carried out with the calibration weight 4.
In the electronic balance having the calibration weight 4 of the pushing-up type mechanism described above, it is necessary to adjust a height of the vertically moving bar 5 abutting against a lower surface 4a of the calibration weight 4 and an engagement between the weight holding member 7 and the calibration weight 4. It is difficult to simplify a process of the adjustment, thereby causing an obstacle for mass production.
Therefore, in a recent electronic balance, as shown in FIG. 7, an elastic member 9 such as rubber is inserted between the weight holding member 7 for pressing the calibration weight 4 and a column 10 for adjusting a relative height between the weight holding member 7 and the vertically moving bar 5. Alternatively, the vertically moving bar 5 is formed to be extendable for adjusting the relative height between the weight holding member 7 and the vertically moving bar 5.
The conventional electronic balances have the structures as described above. In the electronic balance wherein the elastic material 9 such as rubber is inserted between the weight holding member 7 and the column 10 for adjusting the space between the weight holding member 7 and the leading end of the vertically moving bar 5 as shown in FIG. 7, it is necessary to adjust the space between the weight holding member 7 and the leading end of the vertically moving bar 5 for each electronic balance. However, it is difficult to adjust the height of the weight holding member 7, thereby causing variation in the height. Also, torque applied to a motor in the sliding cam mechanism 6 may vary for each product, thereby causing variation in sound of the motor.
In the electronic balance wherein the vertically moving bar 5 is extended to hold the calibration weight 4, the structure becomes complicated, thereby increasing manufacturing cost. When the electronic balance is transported, the vertically moving bar 5 tends to be displaced downwardly due to vibration or dropping, so that the load sensor may be damaged.
In view of the problems described above, it is an object of the invention to provide an electronic balance wherein the problems described above are solved.
Further objects and advantages of the invention will be apparent from the following description of the invention.